The present invention generally relates to improved current-steering D/A conversion, and particularly to an improved method for using a current-steering D/A converter, and to the D/A converter. The method and the converter specifically comprise compensation for deterministic errors due to linearly graded current source mismatch in the D/A converter.
D/A converters are commonly used in integrated circuits made by CMOS technology, but may also be used in other types of technologies.
D/A converters can be implemented in a variety of ways. For reasons of technology and precision many converters use parallel-connected current sources whose output is directed either towards an output of the converter or towards a reference terminal. The current sources are typically formed by multiple current mirror whose output transistors are all preferably identical.
Such D/A converters are depicted in for example U.S. Pat. Nos. 5,870,044, 5,162,800, 5,870,044 and 5,105,193.
Mismatch between current sources is a crucial problem in current-steering D/A converters for high-speed and high-resolution applications. Today, complicated layout styles or randomization or dynamic element matching (DEM) techniques are used to solve the problem.
Accordingly, it is an object of the present invention to provide a method for using a current-steering N-bit D/A converter comprising N binary weighted current sources SIk, k=0, 1, . . . , Nxe2x88x921, connectable to a common output, each current source SIk comprising 2k unit current sources, SIunit, of equal strength connected in parallel, wherein digital input bits bi, i=0, 1, . . . , Nxe2x88x921, bNxe2x88x921 being the most significant bit (MSB), determine which respective current source, SIk, k=0, 1, . . . , Nxe2x88x921, to be connected to the output, which solves, or at least reduces, the problem of mismatch between current sources as depicted above.
It is in this respect a particular object of the invention to provide such a method that is simple, fast, accurate, precise, effective, reliable, and easy to install, and particularly of low cost.
It is yet a further object of the invention to provide such method, which may be implemented by using a minimum of components.
These objects among others are, according to first aspect of the invention, attained by a method, in which, during D/A conversion, the current INxe2x88x921 from the largest current source SINxe2x88x921 is substituted for a current ĨNxe2x88x921, where             I      ~              N      -      1        =            (                        ∑                      k            =            0                                N            -            2                          ⁢                  xe2x80x83                ⁢                  I          k                    )        +          I      unit      
Ik being the current from the current source SIk, and Iunit being the current from an additional unit current source.
The method is denoted simple MSB (Most Significant Bit) calibration, as it only compensates for the current INxe2x88x921 from the largest current source SINxe2x88x921. The method is preferably implememted such that the D/A converter is calibrated prior to D/A conversion. The calibration comprises that the currents INxe2x88x921 and ĨNxe2x88x921 are measured and that the current difference xcex94I between the measured currents is formed and stored. The substitution performed during conversion comprises then that the current ĨNxe2x88x921 is formed by subtracting the current difference xcex94I from the current INxe2x88x921 of the largest current source.
The above-mentioned objects among others are, according to a second aspect of the invention, attained by a method, in which, during D/A conversion, the currents INxe2x88x921, INxe2x88x922, . . . , INxe2x88x92c, from the c largest current sources SINxe2x88x921, SINxe2x88x922, SINxe2x88x92c, c being a positive integer larger than 1, are substituted for currents ĨNxe2x88x921,ĨNxe2x88x922, . . . , ĨNxe2x88x92c, where                                           I            ~                                N            -            1                          =                  xe2x80x83                ⁢                              (                                          ∑                                  k                  =                  0                                                  N                  -                  c                  -                  1                                            ⁢                              xe2x80x83                            ⁢                              I                k                                      )                    +                      (                                          ∑                                  j                  =                                      N                    -                    c                                                                    N                  -                  2                                            ⁢                              xe2x80x83                            ⁢                                                I                  ~                                j                                      )                    +                      I            unit                                                                        I            ~                                N            -            2                          =                  xe2x80x83                ⁢                              (                                          ∑                                  k                  =                  0                                                  N                  -                  c                  -                  1                                            ⁢                              xe2x80x83                            ⁢                              I                k                                      )                    +                      (                                          ∑                                  j                  =                                      N                    -                    c                                                                    N                  -                  3                                            ⁢                              xe2x80x83                            ⁢                                                I                  ~                                j                                      )                    +                      I            unit                                                            xe2x80x83                ⁢        …                                                      I            ~                                N            -            c                          =                  xe2x80x83                ⁢                              (                                          ∑                                  k                  =                  0                                                  N                  -                  c                  -                  1                                            ⁢                              xe2x80x83                            ⁢                              I                k                                      )                    +                      I            unit                              
in which expressions Ik being the current from the current source SIk, and Iunit being the current from an additional unit current source. This method is denoted generalized MSB calibration, as it compensates for the currents INxe2x88x921, INxe2x88x922, . . . , INxe2x88x92c from the c largest current source SINxe2x88x921, SINxe2x88x922, . . . , SINxe2x88x92c.
The present method may be implemented in the same manner as the method of the first aspect of the invention, but preferably the method is implemented through the following calibration procedure prior to D/A conversion:
The currents INxe2x88x921, INxe2x88x922, . . . , INxe2x88x92c, and ĨNxe2x88x921 are measured;
current difference xcex94INxe2x88x921=INxe2x88x921xe2x88x92ĨNxe2x88x921 is formed; and
current differences xcex94INxe2x88x922=INxe2x88x922xe2x88x92ĨNxe2x88x922, . . . , xcex94INxe2x88x92c=INxe2x88x92cxe2x88x92ĨNxe2x88x92c are provided as fractions of xcex94INxe2x88x921. The substitution, during conversion, comprises that the respective current ĨNxe2x88x921,ĨNxe2x88x922, . . . , ĨNxe2x88x92c, is formed by subtracting the respective current difference xcex94INxe2x88x921, xcex94INxe2x88x922, . . . , xcex94INxe2x88x92c, from the respective current INxe2x88x921, INxe2x88x922, . . . , INxe2x88x92c.
Preferably, the fractions are provided from prior knowledge of the relative mismatch between the N binary weighted current sources.
A further object of the present invention is to provide current-steering N-bit D/A converters, comprising N digital inputs, each receiving a digital input bit bi, i=0, 1, . . . , Nxe2x88x921, bNxe2x88x921 being the most significant bit (MSB); an analog output; and N binary weighted current sources SIk, k=0, 1, . . . , Nxe2x88x921, connectable to said analog output, each current source SIk comprising 2k unit current sources, SIunit, of equal strength connected in parallel, wherein the digital input bits are indicative of which respective current source, SIk, k=0, 1, . . . , Nxe2x88x921, to be connected to the analog output, in which the method according to the first and second aspects of the invention, may be implemented.
Consequently, there is according to a third aspect of the present invention provided such a D/A converter further comprising an additional unit current source and means for substituting the current INxe2x88x921 from the largest current source SINxe2x88x921 for a current ĨNxe2x88x921 where             I      ~              N      -      1        =            (                        ∑                      k            =            0                                N            -            2                          ⁢                  xe2x80x83                ⁢                  I          k                    )        +          I      unit      
Ik being the current from the current source SIk, and Iunit being the current from said additional unit current source.
According to a fourth aspect of the present invention there is provided such a D/A converter further comprising an additional unit current source and means for substituting the currents INxe2x88x921, INxe2x88x922, . . . , INxe2x88x92c, from the c largest current sources SINxe2x88x921, SINxe2x88x922, . . . , SINxe2x88x92c, c being a positive integer larger than 1, for currents ĨNxe2x88x921,ĨNxe2x88x922, . . . , ĨNxe2x88x92c, where                                           I            ~                                N            -            1                          =                  xe2x80x83                ⁢                              (                                          ∑                                  k                  =                  0                                                  N                  -                  c                  -                  1                                            ⁢                              xe2x80x83                            ⁢                              I                k                                      )                    +                      (                                          ∑                                  j                  =                                      N                    -                    c                                                                    N                  -                  2                                            ⁢                              xe2x80x83                            ⁢                                                I                  ~                                j                                      )                    +                      I            unit                                                                        I            ~                                N            -            2                          =                  xe2x80x83                ⁢                              (                                          ∑                                  k                  =                  0                                                  N                  -                  c                  -                  1                                            ⁢                              xe2x80x83                            ⁢                              I                k                                      )                    +                      (                                          ∑                                  j                  =                                      N                    -                    c                                                                    N                  -                  3                                            ⁢                              xe2x80x83                            ⁢                                                I                  ~                                j                                      )                    +                      I            unit                                                            xe2x80x83                ⁢        …                                                      I            ~                                N            -            c                          =                  xe2x80x83                ⁢                              (                                          ∑                                  k                  =                  0                                                  N                  -                  c                  -                  1                                            ⁢                              xe2x80x83                            ⁢                              I                k                                      )                    +                      I            unit                              
in which expressions Ik being the current from the current source SIk, and Iunit being the current from said additional unit current source.
The inventive D/A converters may comprise a current mirror for the forming of above said current differences, which differences may be stored and restored in a network comprising a capacitor and transistors connected in parallel. The transistors comprise preferably both NMOS and PMOS transistors. Further, the current sources are preferably also MOS transistors.
A major advantage of the present invention is that it can be practiced in a completely analog way without introducing any A/D conversion.
Further characteristics of the invention and advantages thereof will be evident from the following detailed description of embodiments of the invention.